Objects such as wafers and photomasks are evaluated by illuminating the object by radiation and detecting signals that are scattered from the object, are reflected from the object or pass through the object. The radiation can include light radiation, ultra violet radiation and deep ultra violet radiation.
The detected signals are processed by one of few well known defect detection algorithms. Some of the algorithms include comparing detection signals that represent an inspected pattern to corresponding signals that represent a reference pattern. The latter may be for example a corresponding similar pattern in another region of the object under inspection (commonly known as “Die-to-Die” inspection). Another example is a synthetic pattern produced by an appropriate physical modeling of a desired design (as in “Die-to-Database” comparison).
In many cases the inspected object includes a repetitive pattern. The repetitive pattern includes multiple regularly repeating structural elements and their background. These multiple regularly repeating structural elements are optically distinguishable from their background. For example, while the multiple regularly repeating structural elements can be opaque the background can be transparent.
With either method, detection signals of the object (that may form an image of the inspected object), are subtracted from reference signals (that may form an image of a reference object), and some simple metric of the difference, such as its peak absolute value, is used for determining whether a defect exists.
The intensities of the detection signals of the inspected pattern can be proportional to the squared absolute (complex) amplitude of the electric fields associated with the inspected object but can, additionally or alternatively, be proportional to the field perturbation due to the presence of the defect.
Defect information representative of defects that appear over regions of the pattern that give rise to low-amplitude image electric field may typically be substantially weaker (and even unnoticeable) in relation to defects that appear over regions of the pattern where the resulting image electric field has a substantial amplitude.
FIG. 1 illustrates a non-limiting example of portion 12 of a repetitive pattern 11 of an object 10. Repetitive pattern 11 includes multiple substantially opaque regularly repeating structural elements such as lines 14(1), 14(2) and 14(3) that are formed over a background such as transparent line 15. A substantially transparent defect 16 is located above (or within) line 14(3) while another substantially opaque defect 17 is located above (or within) background 15. It is expected that defect information relating to defect 16 will hardly be noticeable, or will not be as noticeable as defect 17 that is located at a position in which the value (amplitude, field strength) of the pattern information is high. It is noted that in general, similar defects that are located within different backgrounds can result in different detection signals.
FIG. 2 illustrates prior art detection signals obtained from scanning that portion of the repetitive pattern.
Oscillating curve 24 illustrates the electromagnetic field expected to be formed on a sensor as a result of an ideally defect free reference repetitive pattern. The net electromagnetic field received (at a sensor) at the location of defect 16 is denoted 26 and the electromagnetic field received at the position of defect 17 is denoted 27. On the bottom panel of FIG. 2, the net intensities received at a sensor of defects 16 and 17 are illustrated, denoted 26′ and 27′, respectively.
As can be clearly seen in FIG. 2, the difference in electric field due to defect 16 is much smaller than that due to defect 17. As a result, defect information relating to defect 16 can be hardly noticeable by an inspection process. Hence there is a need for systems and methods that enable defect detection also at positions where the amplitude of the pattern field is small.